1. Field of the Invention
The present invention relates to flash tube or speed-light apparatus, and more particularly to a flash tube drive circuit in a speed-light apparatus.
2. Description of the Prior Art
In flash photography with a single-lens reflex camera having a through-the-lens light measurement system equipped with a speed-light apparatus, it has been proposed to control the quantity of light emitted by the speed-light apparatus by measuring the light emitted by the speed-light apparatus and reflected by the object to be photographed.
In such conventional apparatus, however, a high voltage used for triggering the flash tube often generates noise affecting the light-measuring circuit, and the circuit assumes a "latched" or abnormal state and is unable to provide a correct light measurement output. For this reason the conventional speed-light apparatus is deficient in its ability to properly control the quantity of light emitted and thus to provide an appropriate exposure.
A conventional light measuring circuit comprising an operational amplifier, a light-sensitive element connected between the inverting input and non-inverting input terminals thereof, and a logarithmic diode connected between input and output terminals of the operational amplifier, may be provided with a bypass diode parallel to the logarithmic diode to expedite recovery from the abnormal state caused by the triggering noise. However, such a light measuring circuit is not free of the undesirable latching effect, although recovery from the abnormal state caused by the noise is expedited.